1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a ferroelectric liquid crystal display having reduced flicker and a stable liquid crystal alignment.
2. Description of the Related Art
A liquid crystal display (LCD) controls the light characteristics of a display screen so as to produce a desired image. Liquid crystals used in liquid crystal displays are in a neutral state between a liquid and a solid. That neutral state has both fluidity and elasticity.
While there are many types of liquid crystals, one type of great interest is the smectic C liquid crystal. During a thermodynamic phase transition, smectic C liquid crystal molecules rotate along an outer line of a virtual cone. Such a smectic C phase liquid crystal can undergo a spontaneous polarization. Such a liquid crystal is usually referred to as a “ferroelectric liquid crystal” (FLC). The FLC has been actively studied because of its fast response time. Furthermore, FLC LCDs can have wide viewing angles without the complications of special electrode structures or compensating films.
There are many different FLC modes, including a deformed helix FLC, a surface stabilized FLC, an anti-FLC, a V-mode FLC and a half V-mode FLC. Hereinafter, the V-mode FLC mode and the half V-mode FLC mode will be described in more detail.
FIG. 1 shows a V-mode FLC liquid crystal cell. As shown, that liquid crystal cell includes an upper substrate 1 having a common electrode 3 and an alignment film 5. That liquid crystal cell also includes a lower substrate 11 having a TFT array 9, which includes pixel electrodes, and an alignment film 7. A V-mode liquid crystal 13 is interposed between the upper and lower substrates 1 and 11. The alignment films 5 and 7 are aligned in a horizontal direction, usually by rubbing the alignment layers with a special cloth roller. The V-mode liquid crystal 13 forms multiple smectic layers that have molecular structures arranged with desired slopes with respect to a plane perpendicular to the smectic layers. In other words, the liquid crystal molecules have desired inclination angles with respect to the horizontal alignment direction of the alignment films. Furthermore, adjacent smectic layers have opposite polarities.
Light transmission through a V-mode FLC liquid crystal cell varies according to an applied voltage across that cell, reference FIG. 2. The liquid crystal 13 within the V-mode FLC liquid crystal cell responds to both positive and negative voltages. Since the light transmissivity rapidly changes in accord with applied positive and negative voltages, the light transmissivity verses voltage curve has the V shape shown in FIG. 2. Thus, light transmissivity increases regardless of polarity.
FIG. 3 shows an alignment state of a half V-mode FLC liquid crystal cell. As shown, a half V-mode FLC liquid crystal 15 is interposed between an upper substrate 1 and a lower substrate 11. The half V-mode FLC liquid crystal 15 forms multiple smectic layers in which the liquid crystal molecules align at desired inclination angles with respect to a horizontal alignment direction of the alignment films 5 and 7. However, as shown in FIG. 3, the liquid crystal molecules in adjacent smectic layers have the same polarity (unlike V-mode FLC liquid crystal molecules). Such a half V-mode FLC liquid crystal can be formed by applying a positive (or a negative) electric field across a hot liquid crystal, and, at the same time, lowering that liquid crystal's temperature into a smectic phase.
A half V-mode FLC mode liquid crystal 15 formed in this manner responds to only one polarity of applied voltage. Thus, as shown in FIG. 4, a light transmissivity verse voltage curve of a half V-mode FLC liquid crystal cell has a ‘half V’ shape. Still referring to FIG. 4, as shown, the light transmissivity verses voltage curve does react, slightly, to negative applied voltages, but dramatically to positive applied voltages.
The light transmissivity curve shown in FIG. 4 represents a liquid crystal cell in which the liquid crystal molecules are aligned by a negative voltage. In this case, the light transmissivity of the liquid crystal cell almost does not increase when a negative voltage is applied, but rapidly increases when a positive voltage is applied. On the other hand, a liquid crystal aligned by a positive voltage increases its light transmissivity with an increase in a negative voltage.
The thermodynamic phase transition of a half V-mode FLC liquid crystal 15 is as follows:                Isotropic→nematic (N*) phase→smectic C* (Sm C*) phase→crystal        
Such thermodynamic phase transitions express the phases of the liquid crystal in accordance with temperature, which becomes less as phase changes move to the right.
An isotropic phase liquid crystal 15 interposed into a liquid crystal cell aligns in parallel with the rubbing direction of an alignment layer when the liquid crystal temperature is slowly lowered to the nematic phase. If a sufficiently strong electric field is applied across the liquid crystal cell while the liquid crystal temperature is slowly lowered, the liquid crystal 15 is phase-changed into a smectic phase in which the direction of spontaneous polarization of the liquid crystal molecules is in accord with the electric field. Consequently, when the liquid crystal 15 within the liquid crystal cell is subjected to a parallel alignment treatment, the liquid crystal molecules arrange in a spontaneous polarization direction that is consistent with the electric field at the phase transition, and in one of two possible molecular arrangements. As a result, the liquid crystal 15 has a uniform alignment state.
FIG. 5 and FIG. 6 help illustrate this. First, as shown in FIG. 5, if a negative electric field E(−) is applied during alignment of the liquid crystal 15, then the spontaneous polarization direction of the liquid crystal 15 is along the electric field. In such an aligned liquid crystal cell, as shown in FIG. 6, the liquid crystal arrangement is changed by an applied positive electric field E(+), while a liquid crystal arrangement is not changed by an applied negative electric field E(−).
To utilize the response characteristics of the liquid crystal 15, perpendicular polarizers are arranged on the upper and lower portions of the liquid crystal cell. The transmission axis of one of the polarizers is along the direction of the initial liquid crystal alignment. Assuming a liquid crystal cell having the transmission curve of FIG. 4, an applied negative electric field E(−) does not change the liquid crystal arrangement and the perpendicular polarizer blocks light. A positive electric field E(+) rotates the liquid crystal alignment such that light transmission increases.
As described above, the half V-mode FLC liquid crystal cell uses both temperature and an applied electric field during alignment. However, such a liquid crystal cell has a problem in that breaking the initial alignment, such as by external impacts that are inevitably applied by grinding a shorting bar, destroys that alignment. To re-establish alignment, both temperature and electric fields must be used. However, this is difficult to do once the shorting bar, which was used for applying the electric fields, is removed. In addition to shock, heating a conventional half V-mode FLC liquid crystal cell can destroy the alignment.
Furthermore, the conventional half V-mode FLC liquid crystal cell has a 30 Hz flicker in which light is transmitted at 30 Hz with respect to an alternating current (AC) driving signal of 60 Hz, while light is not transmitted with respect to the remaining driving signal. Accordingly, it is necessary to address the flicker problem to image a stationary picture with an acceptable gray level.
One approach to flicker is to divide the pixel area into two sections having contrary electric charges. However, this scheme is complex to implement, particularly with high brightness.
An alternative scheme for addressing the flicker problem includes increasing the driving signal frequency. In other words, it may be possible to obtain a 60 Hz transmission characteristic (in comparison to the 30 Hz) by driving the display at 120 Hz (instead of at 60 Hz). However, this requires development of a novel driver IC, and thus increases cost.
Another approach to reducing flicker is to use the FLC liquid crystal cell shown in FIG. 7. Referring now to FIG. 7, that FLC liquid crystal cell has the uniform alignment characteristics of a half V-mode FLC liquid crystal cell. Also, the illustrated FLC liquid crystal cell has the symmetrical driving characteristics of the V-mode FLC liquid crystal cell, reference FIG. 8. The illustrated FLC liquid crystal cell has symmetrical driving characteristics because the liquid crystal is positioned at a central portion of a virtual cone area, which represents the rotatable positions, in accordance with temperament or rubbing of the alignment film. The illustrated liquid crystal cell enables the primary alignment direction of the liquid crystals to be consistent with the rubbing treatment of the alignment films.
However, the FLC liquid crystal cell illustrated in FIG. 7 is highly sensitive to the process condition of the alignment film. Since the FLC liquid crystal cell illustrated in FIG. 7 has a small alignment tolerance, the temperament conditions and the rubbing of the alignment film are critical. As a result, it becomes difficult to achieve equal alignment forces on the upper alignment film and on the lower alignment film. Furthermore, because of problems with achieving uniform alignment, the FLC liquid crystal cell illustrated in FIG. 7 has proven difficult to mass produce, particularly in large dimension LCDs.